The invention relates to a circuit arrangement for operating a discharge lamp, provided with
input terminals for coupling to a supply voltage source, PA1 means I coupled to the input terminals for generating a current through the discharge lamp from a supply voltage delivered by the supply voltage source, PA1 a control circuit for controlling the operational state of the means I, comprising PA1 means D coupled to the comparator means Comp for generating a second internal reference signal, PA1 means IV for changing the amplitude of an electrical quantity G present at the input terminal T from a first to a second value, and for changing the operational state of the comparator means Comp from a first state to a second state, said comparator means Comp comparing the internal signal S.sub.int with the first internal reference signal in the first state and comparing the internal signal S.sub.int with the second internal reference signal in the second state.
means II for generating a signal S which is a measure of the value of an operating parameter, PA2 an integrated circuit provided with
an input terminal T coupled to the means II, PA3 means C for generating a first internal reference signal, PA3 means E coupled to input terminal T for generating an internal signal S.sub.int derived from the signal S, PA3 comparator means Comp provided with an output, a first input coupled to the means C, and a second input coupled to the means E, and PA3 means III coupled to the output of the comparator means Comp for changing the operational state of the means I.
Such a circuit arrangement is known from U.S. Pat. No. 4,952,849 (hereby incorporated by reference). The signal S in the known circuit arrangement, which is a measure of the value of an operating parameter, is a signal which is a measure of the current through the discharge lamp (also referred to the lamp hereinafter). The control circuit achieves that the amplitude of the current through the lamp remains approximately constant. It is also possible to choose an alternative operating parameter such as, for example, the voltage across the lamp. If, for example, the discharge lamp fails to ignite after the circuit arrangement has been switched on, the voltage across the discharge lamp will rise to the point where the internal signal S.sub.int exceeds the value of the first internal reference signal. This activates the means III. The means III effect a change in the operating state of the means I such that this causes the voltage across the discharge lamp to drop. It can be achieved thereby that the voltage across the discharge lamp is not maintained at a high value by the circuit arrangement for a long period if the discharge lamp should fail to ignite. It is desirable in many cases not only to provide measures which ensure that the value of an operating parameter does not exceed a maximum admissible value (for a long period), but also to ensure that the same operating parameter cannot assume an undesirably low value. If the operating parameter is the voltage across the discharge lamp, a (too) low value of this voltage may point to a (too) high value of the current through the discharge lamp or a defect in a capacitor connected parallel to the discharge lamp. It may also be desirable, for example, to limit the voltage across the discharge lamp to a first maximum admissible value during ignition and to a second maximum admissible value during stationary lamp operation, which latter value must not be exceeded for more than a predetermined time interval. A change in the operational state of the means I is indicated, for example, by switching-off of the means I, if a maximum or minimum admissible value is exceeded. It is possible to design the control circuit such that the means II are coupled to only one input terminal of the integrated circuit, and that one and the same signal S.sub.int is compared with a first internal reference signal and with a second internal reference signal. Such a construction of the control circuit, however, makes it necessary to lay down the relationship between the two internal reference signals in the integrated circuit. The relationship between the two reference values, for example, the relationship between the minimum admissible value and the maximum admissible value of the operating parameter, is then also laid down in such a construction of the control circuit. The minimum admissible value and the maximum admissible value of an operating parameter, such as the voltage across the discharge lamp, however, are generally speaking dependent on specific properties of the discharge lamp and of the construction of the circuit arrangement. It is desirable for this reason to have a mechanism for setting these reference values independently of one another. This is possible through the use of two input terminals of the integrated circuit. These two input terminals are coupled to the means II in that case, and each of the input terminals is coupled to comparator means and means for generating a reference signal. Such a solution, however, requires a comparatively large number of input terminals, which renders the integrated circuit large and expensive. In addition, this solution often requires a comparatively large number of external components, so that the circuit arrangement becomes complicated and expensive.